1. Field of the Invention
The present invention relates to lumber planing devices of the two-way thicknessing type which plane a piece of lumber on two opposite sides to produce a smooth finished piece of a predetermined thickness.
2. Description of the Prior Art
A typical prior art device for planing both sides of a rough sawn piece of lumber has an infeeding mechanism feeding the workpiece across a lower platen which supports it as it travels beneath an upper planing head which surfaces the top of the workpiece. The workpiece then continues on toward a lower planing head which surfaces its bottom. If the workpiece is not held up against an overhead plate, too much may be planed off the lower surface of the workpiece or the lower surface may be planed only in intermittent patches. Up to now, the only way to avoid these occurrences, both of which will adversely affect the grade of the lumber, was to hold the workpiece against an upper platen as it moves over the lower planing head by a hinged end portion of the lower platen which is urged upward by pneumatic or other means.
However, the provision of a hinged segment on the lower platen is in itself a source of problems. First, the hinge creates a crevice in which splinters and pitch accumulate, causing the workpiece to bend and stick as it passes over the lower platen. Accumulated debris also interferes with the setting of the desired lower to upper platen distance, resulting in inaccuracy in the dimensions of the finished product. Furthermore, the debris can scratch the lower surface of a workpiece, reducing the grade of lumber produced from it.
Another problem associated with a hinged and segmented lower platen is excessive wear caused by platen chatter as uneven portions of a workpiece move over the hinged segment, causing its parts to rattle and impact against one another. This also results in an undesirable noise level. Furthermore, horizontal deflection of the lower platen, causing shifting of the workpiece and strain on the lower platen, results from the hinged mounting of the lower platen.